Evohome - phantom demand from Open Therm.

As per title, system has been installed and working fine for last couple of years.

Ideal logic combi boiler, evohome and 2 HCE80.

Middle of the night the boiler powers on as 15% heat demand from Open Therm. Now rhyme or reason especially as it was such a warm night.

Ended up pulling batteries & reinstating them in the controller to eventually resolve.

Any thoughts on what could have triggered this 15% heat demand?

Thanks